Discussion
Turbine disks are commonly subject to high cycle fatigue failure due to resonant vibration and fluid-structure instabilities. Disks have several critical speeds wherein operation of the disk at any one of these speeds creates an amplified traveling wave within the disk, inducing potentially excessive dynamic stresses. At each of these critical speeds the wave is fixed with respect to the housing and can be excited by any asymmetries in the flow field. The resulting resonant vibration prevents the operation of conventional turbine disks at critical speeds. Fluid-structure instabilities arise due to coupling between the surrounding fluid and the disk, which can also induce excessive stresses and prevent operation at speeds above a threshold stability boundary.
In conventional turbine disks with separate blades assembled onto a disk, blade damping techniques are typically employed to reduce resonant response as well as to prevent the fluid-structure instability that results from the coupling of aerodynamic forces and structural deflections. Accordingly, it is common practice to control blade vibration in the gas turbine and rocket engine industry by placing dampers between the platforms or shrouds of individual blades attached to the disk with a dovetail or fir tree. Such blade dampers are designed to control vibration through an energy dissipating friction force during relative motion of adjacent blades in tangential, axial or torsional vibration modes. Blade dampers, in addition to the blade attachments, provide friction dampening for both disk and blade vibration.
This damping mechanism, however, is not feasible for integrally bladed turbine disks (blisks) unless radial slots are machined between each blade to introduce blade shank flexibility. The added complexity of the slots increases the rim load on the turbine disk and defeats some of the cost, speed and weight benefits of the blisk. Consequently, the lack of a blade attachment interface results in a significant reduction in damping and can result in fluid-structure instability at speeds other than the disk standing wave critical speeds.
Rim dampers have been utilized by the gear industry to reduce vibration in thinly webbed large diameter gears. In such applications a split ring or series of spiral rings are preloaded in one or more retainer grooves on the underside of the gear rim. At relatively low rim speeds the centrifugal force on the damper ring provides damping due to relative motion when the gear rim experiences vibration in a diametral mode. This method of friction damping, however, is not feasible at high rim speeds because the centrifugal force on the damper ring is of sufficient magnitude to cause the damper to lock-up against the rim. Lock-up occurs when the frictional forces become large enough to restrain relative motion at the interface, causing the damper ring to flex as an integral part of the rim.